The attention of the country zeroed in on chemical leaks at Union Carbide plants in West Virginia this week. But some less-noticed toxic-chemical spills from container vehicles in other parts of the United States have underscored another problem that experts say poses a greater threat to public safety than the possibility of a chemical plant leak.

So far this week, shipments of hazardous cargo have made headlines in three states:

In Fairfax County, Va., a tank truck leaked 500 gallons of a highly corrosive chemical onto the Capital Beltway, surrounding Washington, D.C., closing down part of the thoroughfare and causing 630 nearby residents to flee their homes.

In Camden, N.J., a truck spilled 2,500 gallons of the highly toxic chemical analin into a city sewer. More than 100 people had to be evacuated.

These shipments represent a minuscule fraction of the hazardous substances carted daily across the nation's roads and railways. Estimates vary from 100,000 to 250,000 shipments a day, and federal Department of Transportation (DOT) figures put the total annual tonnage of dangerous cargo at 4 billion tons annually.

Despite the size and potential hazard of the shipments, hazardous-materials transportation is an issue that has been largely left alone by consumer and environmental groups.

``The transportation issue is one of the most neglected areas,'' says David Doniger, an official of the Natural Resources Defense Council. ``That is one reason why there is a lot of potential harm.''

Though statistics are somewhat fuzzy, most experts agree that the number of incidents involving hazardous materials has been dropping. DOT statistics recorded 9,063 incidents in 1981, and only 5,512 last year.

Yet critics of the nation's transportation industry say the potential for disaster still exists. They contend that too many aspects of accident prevention -- such as the choice of transportation routes, specifications of toxic-substance containers, and emergency response procedures -- are poorly enforced by the federal government, poorly regulated, or have no laws covering them to begin with.

A Freedom of Information Act request by the Environmetal Policy Institute last year disclosed that the DOT had never enforced a 1939 law requiring that truckers choose the safest, rather than the quickest, route for transporting their cargo. So several cities, such as Denver, New York, and Boston, as well as the State of New Hampshire, have used DOT seed money to establish permit systems to finance their own enforcement efforts.

For its part, the DOT is commissioning several studies to look at storage-tank design used to transport hazardous chemicals such as gasoline -- considered a prime source of hazardous-substance accidents.

The 1974 Federal Hazardous Materials Act is the foundation upon which federal, state, and local regulations are built. It is up for congressional reauthorization next year.